1. Field of the Invention
The present invention relates to heating and cooling systems and, more particularly, to such systems which include a heat pump unit operatively associated with auxiliary heat exchange means.
2. Description of the Prior Art
A heat pump is essentially a device for pumping an appropriate refrigerant fluid around a closed circuit for the purpose of heating or cooling a generally indoor space. The conventional elements of a heat pump include a compressor, an expansion valve, an indoor heat exchange coil, an outdoor heat exchange coil, a refrigerant fluid, suitable refrigerant piping, and a refrigerant flow reversing valve. The heat pump has two sides--a low pressure side and a high pressure side. This pressure difference is caused by the compressor and expansion valve which also separate the two sides. One heat exchange coil is located on one pressure side while the other heat exchange coil is on the other side. Generally one heat exchange coil is located inside an enclosure to be heated or cooled and the other coil is located outdoors. The reversing valve is used to reverse the direction of the flow of refrigerant through the heat pump which has the effect of reversing the pressure sides. Thus, at one time the inside coil can be on the low pressure side while at another time the outside coil can be on the low pressure side. Heat is absorbed by the refrigerant in the coil on the low pressure side and given up by the refrigerant in the coil on the high pressure side. Thus, a heat pump transfers heat between the indoor and outdoor coil depending on the position of the reversing valve.
When used as a refrigerating or an air conditioning device, the inside heat exchanger is located on the low pressure side and within the space to be cooled. Heat is absorbed by the liquid refrigerant evaporating within the inside heat exchanger and is rejected by the vaporized refrigerant condensing in the outdoor heat exchanger. Thus, during hot weather, heat is moved from indoors to outdoors to cool the enclosure. When used as a heating device, the inside heat exchanger is located on the high pressure side and within the space to be heated. Heat is absorbed by the liquid refrigerant evaporating within the outdoor heat exchanger and is rejected by the vaporized refrigerant condensing in the inside heat exchanger. Thus, during cold weather, heat is moved from outdoors to indoors to warm the enclosure.
One noteworthy shortcoming of a conventional het pump is its inability to transfer sufficient heat from outdoors to indoors to warm the enclosure during very cold weather when outdoor ambient temperatures are very low. As a practical matter, when the outdoor ambient temperature falls below about 35.degree.-45.degree. F. there is a notable reduction in the capacity of the outdoor heat exchange coil to provide satisfactory heating. This is, in large part, due to the decreased heat which can practically be absorbed by the coil at very low outdoor ambient air temperatures. When the outdoor ambient temperature drops and evaporation is accomplished in an outdoor air heat exchanger of fixed geometry, the result is a drop in evaporation temperature and pressure. This causes a substantial reduction in the density of the refrigerant vapor. The compressor, therefore, can circulate only a substantially reduced mass of refrigerant which accounts, in part, for the substantially reduced heating capacity of the system. Moreover, at the reduced refrigerant pressures, there is a marked loss of volumetric efficiency of the compressor both in terms of quantity of heat contributed by the compressor and in relative heat contribution to the refrigerant fluid.
The practical solution of this requirement for additional heat for the indoor space to be heated has been to furnish supplemental heating, usually in the form of relatively expensive electrical resistance heating or, alternatively, fossil fuel heating. However, with decreasing availability of fossil fuels, increasing energy costs and demanding space and health considerations, neither of these solutions is very appealing or practical any longer. Instead, supplemental heat for heating the indoor space is now frequently derived from a third heat exchange coil disposed in heat exchange relationship with a stable temperature heat source, such as ground water or heat storage facilities which are thermally charged from any of a variety of thermal sources, such as solar collectors, electrical resistance heaters operated during off peak, low demand hours or even from the heat pump unit itself operated during periods of relatively high ambient air temperatures. Such an arrangement is illustrated, for example, in U.S. Pat. No. 4,165,037 which discloses an auxiliary heat exchanger operatively associated and in parallel with respect to refrigerant flow with the outdoor heat exchanger of a heat pump unit. During periods of severely low outdoor ambient temperature, when the efficiency and capacity of the outdoor heat exchanger is reduced and impaired, refrigerant flow is diverted to the auxiliary heat exchanger which derives its thermal energy from a water storage source heated by a solar collector unit. A somewhat similar arrangement is disclosed in U.S. Pat. No. 4,256,475 which shows a solar heated water storage unit for supplying water to the coil of an auxiliary water heat exchanger arranged in parallel with the outdoor heat exchanger of a heat pump unit. When the heat pump unit, due to low outdoor ambient temperature, cannot transfer sufficient heat from the outdoor air to warm the space to be heated, water from the storage unit is circulated to the coil of the auxiliary water heat exchanger to carry heat from the solar heated water storage unit to the refrigerant and, eventually, via the indoor heat exchanger to the space to be heated. Also of interest is U.S. Pat. No. 3,563,304 which discloses an auxiliary heat exchange coil in heat exchange relationship with a pool of water arranged in series with the conventional outdoor coil of a heat pump unit. During the heating mode of heat pump operation a refrigerant first absorbs heat from the outdoor ambient air in the outdoor heat exchange coil and then absorbs heat from the pool of water in the auxiliary heat exchange coil. However, when outdoor ambient air temperature is extremely low and it is desired to remove the conventional outdoor coil from operation, by virtue of the series arrangement refrigerant will still pass through the outdoor coil and heat will be lost therein.